The androgen receptor (AR) is known to be expressed in the majority of estrogen receptor (ER) alphapositive human breast tumors. By gene expression profiling, we discovered elevated AR RNA in clinical breast tumors resistant to antiestrogen therapy with tamoxifen (Tarn). We have shown that overexpression of AR causes ER alpha-positive MCF-7 breast cancer cells to become resistant to the growth-inhibitory effects of Tam and to estrogen withdrawal, which models the therapeutic action of aromatase inhibitors [Als]). This endocrine resistance could be reversed by the AR antagonist bicalutamide, or by AR knockdown. Furthermore, in AR-overexpressing breast cancer cells, Tam induced rather than repressed ERalpha's transcriptional activity, and this could also be reversed by bicalutamide. We therefore hypothesize that AR overexpression is a novel mechanism of resistance to ER-targeted therapies. We have developed this translational study to extend these findings, to determine how AR causes resistance to Tam and Als and thus identify potential predictive markers for resistance and possible intermediate targets for reversing resistance, and finally to test this hypothesis in an initial clinical trial using bicalutamide to restore response in breast cancer patients whose tumors become resistant to Tam or Al treatment. Our proposed Aims are: (1) To determine the contribution of AR crosstalk with growth factor receptors and ER alpha in the resistant phenotype associated with AR overexpression using various signal transduction inhibitors and cell biological assays. (2) To determine how AR overexpression causes Tarnmediated nuclear ER transcriptional activation, exploring genomic AR actions. (3) To examine how AR overexpression affects survival pathways during estrogen deprivation with an Al. (4) To determine whether the AR antagonist bicalutamide can reverse endocrine resistance in breast cancer patients progressing on Tam or an Al in a Phasei/ll clinical trial. These studies will employ techniques to explore the molecular mechanisms of AR action in breast cancer cells, which is an understudied area. We will identify whether specific components of the AR signaling pathway can be exploited to reverse endocrine resistance. We anticipate that AR will become an important new marker of endocrine resistance, and with the availability of an FDA-approved agent to block its effects (bicalutamide), we can rapidly translate our results into a possible new strategy for maintaining the benefits of endocrine therapy in breast cancer patients.